Spoiler (click to reveal)
At the end, when it says "This card may be one of the cards used." If you build a Supply Center on a blue space, spending the Paris card allows you to spend 1 less card, as if the card was still in your hand.
You can't spend it to build a supply center on a different color space, like yellow. However, keeping the card in your hand will allow you to spend 1 less card for Supply Centers on another space.